Many questions on StackOverflow relate to the current status of a particular language, component, or software tool (current version.) The majority of questions over 18? months old can be called into question as they are outdated.

It's highly likely that a new version of the specific language, component, or software tool has been released since the time of the question and the answer may no longer apply. Often you obtain the wrong answer on StackOverflow as the question is old and the missing/broken functionality has been addressed since that time.

Tags could be more version-dependant as a softening agent but it's a generic problem of all of these sites. A new user of Stack Overflow could easily come to the site with a particular question and be provided with an answer that was voted up and selected as the best answer only to find out later that it's no longer the case. Or, worse, they may never find out later that the answer has changed and they help contribute to the ongoing fud about a product/tool...

All answers are only considered best at the time of the response. All answers are subject to scrutiny over time. In the least, "old" answers could be highlighted differently to warn the user of the length of time the answers were provided. It's definitely debatable what constitute "old" information so perhaps that would be configurable by tag moderators for lack of a better idea.

Wow...lot of negative with no comments. Downvoters are ignoring a large problem with these sites...what percentage of questions over 2 years old are still valid? How many people monitor new questions versus editing old questions? What is the incentive of editing old questions? If there shouldn't be duplicate questions asked, how do you properly reconcile new features in tools/software/components? (What percentage of questions specify a specific revision #?)
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Darian MillerDec 30 '11 at 16:08

3 Answers
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I strongly disagree with even with the core contention here, that the majority of questions older than 18 months are totally out of date and no longer relevant. I frequent iphone, a platform that has seen four major OS updates in the last three years I've been here, in one of the fastest moving areas in modern computing, and yet almost all of the older upvoted questions and answers I see are still perfectly relevant.

I regularly find myself coming upon questions and answers much older than 18 months when searching for solutions to problems I'm facing. Were those to be automatically deleted or suppressed by the system past a certain age, not only would I not be able to find the solution, the work put in by people to provide definitive references for these topics would be destroyed.

When it comes to answers that are no longer correct, comments usually indicate this, and the answers either are subsequently downvoted and replaced at the top by something more modern, or they are edited. A prime example of the latter is Adam Davis' answer about sending SMS messages on the iPhone. At the time it was written, there was no way to do this. However, since then two OS updates have added built-in SMS capabilities and the answer has been edited to reflect this.

Finally, for questions that were specific to a particular point in time that has now passed, we have the "too localized" close vote reason. This gives the community the ability to identify content that will no longer be of use to people in the future and selectively remove it, rather than applying an arbitrary expiration date to every piece of content.

There's plenty of wiki-like features to help keep things up-to-date, so this isn't so much a concern. Dropping questions for reasons of age only is problematic for legacy systems, and things that don't change, but are still very widely used. This isn't a great way to deal with it. If you're consistently running into issues where information is flat wrong, then follow one of the below suggestions to resolve that issue, for you and future users.

The wiki/community features take care of the rest of the concerns. Any users with enough reputation can edit any answer, forever in the future. If an answer becomes truly irrelevant, either a new answer can be added, or the original answer can be modified to adapt to the new information. Again, users with enough reputation can edit the answer to note that the information is out of date, for example, to add a note to the effect of, "This information is only relevant to versions 1.x-3.2" or "this API call has been deprecated," etc.

Finally, you can also deal with this specific problem by clicking on "newest" on your search results to see the most recent answers. If you're searching on things that are actively developed and being changed, a simple glance at the date upon which the question/answer was posted will give you some idea of its "freshness" - it's part of the data that's listed on every question and answer.